A. Field of the Invention
This invention relates generally to the field of couplings for joining pipe segments together. The term "no-hub" in the context of couplings refers to pipe segments that have no spigots or other features at their ends. No-hub couplings are often used for coupling pipe segments in drain, waste and vent piping systems.
B. Discussion of Related Art and Improvements of the Invention
General background information on no-hub couplings is described in the Cast Iron Soil Pipe Institute standard No. 310-90. This standard describes a pipe coupling having a shield with a plurality of corrugations parallel to the axis of the pipe, two or four clamping bands covering the shield, and an elastomeric gasket covered by the shield which makes sealing contact with the pipe segments.
No-hub couplings were introduced into the United States in the early 1960's. An early patent describing a no-hub coupling is the patent to Evans, U.S. Pat. No. 3,233,922. The no-hub coupling described in the Evans patent suffers from a low resistance to pull-out forces. These pull-out forces occur under two situations: (1) an accidental stoppage in the system, or (2) an intentional stoppage produced once, at either the completion of construction, or by a plumbing inspector to verify freedom from leakage in the system.
In the early 1980's heavy-duty couplings were introduced to the market to improve the resistance to pull-out forces in the coupling. One of these couplings was produced by the Anaheim Foundry Company and is described in their patent, U.S. Pat. No. 4,564,220. Another heavy-duty coupling introduced in the 1980's was known as the "Tyler" pipe coupling from the Plessey Corporation, described in U.S. Pat. No. 4,538,839. Both of these heavy-duty coupling designs increase the strength of the coupling by adding additional bands, in addition to other changes. The standard no-hub couplings were made in sizes of 11/2 inches, 2 inches, 3 inches, and 4 inches, with two clamp bands per coupling. The 5 inch, 6 inch, 8 inch and 10 inch sizes were made with four clamps per coupling. These new heavy duty couplings increase the number of clamps from 4 to 6, respectively.
The industry has established certain tests to be performed on pipe couplings in order to satisfy minimum industry requirements. These tests are the hydrostatic test, the deflection test, the shear test, and the thrust test. The most difficult of these tests to pass is the thrust test. Essentially, the thrust test is an unrestrained "pull-out" test. The thrust test is run by increasing the internal pressure in a water-filled pipe assembly, in timed, incremental steps. When the slippage of the pipe out of the coupling reaches 0.150 inches, the coupling is deemed to have failed. The thrust test standard is described in the Cast Iron Soil Pipe Institute Standard mentioned previously.
The thrust requires that one of the pipes be at a minimum permissible diameter and the other pipe at a maximum permissible diameter. The difference in the size of the pipes during the test is that, in practice, pipes are made to certain tolerances and some variation in the size of the pipes is inevitable. The bilateral tolerance between the two pipes is generally plus or minus 0.09 inches. Therefore, the diameter difference between the two pipe segments is up to 0.18 inches. The disparity in circumference between the maximum and minimum pipes is 0.57 inches. In order to pass the thrust test in these conditions, two things are required. First, the joint must not leak, and second, the rate at which the pipe moves out of the coupling must be minimized.
The above-described pipe couplings rely upon friction alone between the coupling gasket and the pipes to hold the pipes together and prevent pull-out of the pipe from the coupling. In these designs, the clamp bands, when tightened, press down on the shield. The shield, in turn, presses down on the gasket, which is then pressed against the pipe. Any interference with the transmission of the clamp pressure through the shield to the gasket and on to the pipe will result in diminished friction and hence a diminished resistance of a pipe from being pulled out from the coupling.
There are two sources of possible interferences which can result in pipe to coupling displacements, and hence leakage. These possible interferences are at the pipe-to-gasket interface and the gasket-to-shield interface. The inherently higher friction between the pipe and gasket generally prevents movement between these two components at the pipe-to-gasket interface. The displacement of the pipe relative to the coupling is largely caused by the relatively low friction between the smooth stainless steel shield and the gasket.
As mentioned previously, the above described prior art couplings rely on friction alone to hold the two pipes together. It has been discovered that certain inherent design deficiencies in the standard prior art no-hub couplings make these couplings inherently prone to failure of the thrust test, particularly due to the shape in which the shield is maintained when the pipes are clamped onto two different pipe segments of different diameters. The shape of the shield in prior art couplings tends to cause the coupling to suffer from pull-out failures. Moreover, it has been discovered that inherent limitations in the design of prior art coupling shields has promoted a wrinkling of the gasket when the shield is deformed to conform to the smaller diameter pipe size. This gasket wrinkling promotes leakage failures.
With regard to the above-described problem of wrinkling of the gasket which promotes leakage failures, the wrinkling occurs during the process of installing the coupling on to the pipe segments, particularly when one of the pipe segments is of a slightly smaller pipe diameter relative to the other pipe segment, e.g., 0.18 inch diameter difference for a six inch pipe. The wrinkling can become most severe if the two clamps on the larger pipe are tightened before the clamping bands on the smaller pipe segment are tightened. When the shield is clamped about the smaller diameter pipe segment, the shield itself must absorb the circumferential reduction, causing a wrinkling of the shield. If the wrinkling of the shield is distributed evenly throughout the circumference of the shield the coupling will typically not leak. However, if all or most of the wrinkling is localized, it produces a bunching up of the shield, and will result in leakage at either side of the wrinkle. This is illustrated in FIG. 1, which depicts a clamp band 10 clamping a shield 12 and gasket 14 on to a pipe segment 16. Region A is a region of only minor wrinkling, and region B is a region of pronounced wrinkling, and hence leakage in region C. Our invention substantially eliminates the bunching up of the shield, as shown in FIG. 1, by distributing the circumferential reduction in the shield evenly around the entire circumference of the shield.
In the coupling of the prior art Evan's '922 patent and the coupling shield described in the CISPI Standard mentioned previously, the shield corrugations are oriented parallel to the pipe axis. We have discovered that the properties of these corrugations relative to controlling the gasket wrinkling problem are poor. The corrugations are uninterrupted throughout the length of the shield. Therefore, there is no way of evenly distributing the wrinkles which are formed when the clamps are tightened to collapse the shield about a smaller diameter pipe. As mentioned before, this change in diameter requires approximately a 9/16 inch circumference reduction. That much reduction results in random, sometimes unacceptably large and localized wrinkles. These wrinkles produce low pressure regions on the gasket adjacent to the wrinkles, resulting in leakage. The corrugations in the coupling of the Evans '922 patent provide no mechanical impediment to gasket-to-shield movement. The friction at the gasket-to-shield interface is inherently low due to the smoothness of the steel shield surface.
In the configuration of the Evans '922 patent, the relatively flexible steel shield is stiffened considerably, particularly in the lateral direction. This stiffness is in such a direction so as to force the shield to take on an undesirable conical shape. The conical shape can be seen in FIGS. 2A and 2B, and the shouldered shape provided by our invention can be seen in FIG. 2C. In FIG. 2B, the pipe segment 16 has been shifted over such that the edge 20 of pipe segment 16 is flush with edge 22 of pipe segment 24, a situation which occurs in the field. The problem with the shield retaining the conical shape as shown in FIGS. 2A and 2B is that compressive forces on the gasket are minimized, decreasing the contact between the gasket and the pipe. The conical shape of the shield creates spaces in regions A, B, C, or D, in which little or no compressive force between the gasket and the pipe will exist. The resistance to pull-out is therefore limited to the friction produced by approximately one clamping band instead of two clamping bands. We have recognized this problem, and appreciated that if the shield is collapsed about both pipe segments, such as shown in FIG. 2C, the shield has a "shouldered" shape, and all of the band forces will be imposed directly on the gasket. That additional force will raise the friction and therefore the pull-out resistance.
The present invention, in recognizing and providing solutions to these problems, is a no-hub coupling which provides a minimum leakage under the thrust test, as well as insuring that a maximum clamping pressure is exerted on to the clamping bands to the gasket and pipe interface, thereby avoiding pull-out failures.
Accordingly, an object of the invention is to provide a no-hub coupling with improved sealing capabilities. Another object of the invention is to provide a no-hub coupling which increases the coupling's ability to resist pull-out forces and to maintain a tight and secure clamping and sealing of the coupling to the pipe segments.